DTJ Number 3 September 1987 - Digital Technical Journals

Tbe DECnet-DOS SystemNDU and the virtual device drivers were builtfrom three subcomponents:• A common parser and common message processor(described in the next section)• A small library of subroutines that createremote files and open them using the dataaccess protocol (These subroutines weretaken directly from NFT.)• The programming interface libraryNetwork MailDigital provides network-wide mail services for anumber of its systems. These utilities allow usersto compose messages directly within the mailutility or to use an editor or pre-existing text fileas the source . Text can be sent to one or moreusers on a multitude of systems, even using a distributionlist from another text file .The DECnet-DOS mail utility was completed ina short time by combining the common parserwith the mail utility from the DECnet-ULTRIXsoftware. The unique requirements of a smallsystem did present some problems. In a DECnetnetwork, node addressing is done by numericaddresses. However, users often prefer to usenames, which can be more easily remembered.To facilitate that use, each node maintains a databasethat maps names to addresses. Now, sucha database for a large network would be toolarge for a small PC to keep on disk or searchquickly. Yet the PC users may want to send mailto other users anywhere in such a network. Forexample, Digital has an in-house network withover 1 0, 000 nodes, any one of which can beaddressed by any other. To solve this problem weimplemented the mail-forwarding feature of themail protocol. That feature allows a PC user tosend a message to an unknown node by asking aknown node to forward the message. Thus the PCdatabase keeps a much smaller list of knownnodes, which is considerably more manageable.The project team originally had a requirementfor receipt and storage of network mail on thelocal disks of the PC. Our investigations showed;however, that the engineering cost of developinga background task that could record the receivedmail on disk was very high.Since 1/0 in the MS-DOS system is singlethreaded, the network software, running in thebackground, cannot perform 1/0 while an applicationprogram is performing it. To overcomethis restriction, our PC mail service automaticallytells the receiver of mail from the PC whichlarge system should be sent the replies.Application Layer ServicesApplication Program CommandParsingThe DECnet-DOS product would contain as manyas ten different application programs, includingone each for file access, virtual terminal support,virtual device support, and mail services; andtwo for network management. Our requirementscalled for common, easily translatable messagesand common command parsing, inCluding characterdelete, line and character recall, and abbreviation.These standards were important becauseit would be very costly to have ten differentengineers coding in ten different ways . Such anapproach, without standards, could have introduceddifferences in the applications, whichwould be difficult for the end user to learn andremember.To avoid this problem we designed and implementeda common parser and a common messageand help processor. The common parser isdriven by a parsing command file that supportsabbreviation, character delete, and line recall.The common message processor is also driven bya message fi le and performs fast look-ups of textstrings and blocks. This design greatly reducedthe time to code and debug the utility programsand made their translation to foreign languagesfairly straightforward.Network Ma nagementThe network management architecture in theDNA architecture re quires access to currentparameters, ·counters, and statistics, all kept inthe volatile database. It also needs the parameters,kept in the permanent database, that will bein effect the next time the network software isstarted. Data in both databases should be accessiblefrom either the local node or a remote node.However, the single-threaded restriction thatthe MS-DOS system imposes on file 1/0 makesaccess to the network management databasesvery difficult. This restriction not only affects themail service, as explained earlier, but preventsDECnet-DOS from providing remote access tolocal network management databases. It also preventsthe background network software fromaccessing disk files. Thus the project team felt114Digital TecbnicalJournal·No. 3 September 1986